Re: [PATCH 1/4] media: si2168: use bits instead of bool for flags

Mauro Carvalho Chehab
2019-10-10T11:34:30Z

Em Thu, 10 Oct 2019 12:55:44 +0200
Gon Solo <gonsolo@gmail.com> escreveu:

> On Fri, Oct 04, 2019 at 10:15:22AM -0300, Mauro Carvalho Chehab wrote:
> > Using bool on struct is not recommended, as it wastes lots of
> > space. So, instead, let's use bits.
>
> Wouldn't "bool b:1;" even be better? I performed a little test:
>
> #include <stdbool.h>
> #include <stdio.h>
>
> struct uints {
> unsigned int a0;
> unsigned int a1;
> unsigned int a2;
> unsigned int a3;
> unsigned int a4;
> unsigned int a5;
> unsigned int a6;
> unsigned int a7;
> };
>
> struct bools {
> bool a0;
> bool a1;
> bool a2;
> bool a3;
> bool a4;
> bool a5;
> bool a6;
> bool a7;
> };
>
> struct bit_uints {
> unsigned int a0:1;
> unsigned int a1:1;
> unsigned int a2:1;
> unsigned int a3:1;
> unsigned int a4:1;
> unsigned int a5:1;
> unsigned int a6:1;
> unsigned int a7:1;
> };
>
> struct bit_bools {
> bool a0:1;
> bool a1:1;
> bool a2:1;
> bool a3:1;
> bool a4:1;
> bool a5:1;
> bool a6:1;
> bool a7:1;
> };
>
> int main() {
> printf("bit_uints: %ld\n", sizeof(struct bit_uints));
> printf("bit_bools: %ld\n", sizeof(struct bit_bools));
> printf("uints: %ld\n", sizeof(struct uints));
> printf("bools: %ld\n", sizeof(struct bools));
> }
>
> Result:
>
> bit_uints: 4
> bit_bools: 1
> uints: 32
> bools: 8
>
> I know with different types within the struct it looks different, but
> still.

No. In practice, the compiler will add 3 bytes of pad after bit_bools
(on 32-bit archs), due to performance reasons.

Using "unsigned int" makes it clearer.

Thanks,
Mauro